It is known that a vibration sensing type knock sensor that senses vibration of the internal combustion engine is used as a means for identifying a combustion condition in the combustion chamber of the internal combustion engine. Although this knock sensor is intended to sense vibration of the internal combustion engine, it indirectly senses a combustion condition in the combustion chamber and senses that the internal combustion engine vibrates with an excessive increase of the pressure in the combustion chamber, typically because combustion condition changes.
Stated differently, it is thought that the pressure of shock waves resulting from combustion gas in the combustion chamber is transmitted through a cylinder head and pistons. Because the rigidity of the combustion chamber walls of the cylinder head is generally higher than that of the pistons and related parts, most of vibration energy that is transmitted flows from the pistons to the cylinder block. This vibration energy is sensed by a knock sensor which is a vibration sensor. Thus, this vibration represents a fluctuation in combustion condition and, therefore, can be regarded as a combustion noise.
A technique that identifies or detects the occurrence of such a combustion noise and controls combustion so as to suppress the combustion noise is disclosed in Japanese Unexamined Patent Application Publication No. 2010-216264 (Patent Literature 1).
In a flowchart (see FIG. 10) disclosed in Patent Literature 1, based on a combustion temperature which has been read in at S1, an integrated value of knock sensor signals corresponding to frequencies 8 kHz and 6 kHz is detected at S2 and S3. At S4 and S12, a combustion period is determined using the integrated value of the know sensor signals. At subsequent S5, S7, S13, and S17, a determination of ignition timing is made. Further, at S9, it is determined whether an engine misfire occurs and, at S14, a determination is made as to whether a target rail pressure is satisfied. Based on the combustion period and ignition timing thus determined, the fuel injection quantity (PreQ), the amount of exhaust gas recirculation (EGR), and fuel injection timing (IT) are controlled and the routine is terminated.
In the cited literature 1, there is the following description: it is possible to detect a combustion period and ignition timing accurately by selecting frequencies of sensed signals for calculating the combustion period and ignition timing and, thereby, combustion condition can be feedback controlled to a predetermined condition.